Gold plating is generally applied on the surfaces of electronic parts such as printed wiring boards, ceramic IC packages, ITO substrates, and IC cards, because of its physical properties such as the electric conductivity of gold, solderability, and jointability by thermocompression bonding, and its chemical properties such as oxidation resistance and chemical resistance. Many of these electronic parts need gold plating on electrically isolated sites. Therefore, for gold plating, an electroplating process is not applicable, but an electroless plating process is suitable.
Widely known conventional techniques for electroless gold plating include an electroless displacement gold plating process for depositing gold accompanied with dissolution of undercoat metal such as nickel and an autocatalytic gold plating process in which a reducing agent having catalytic activity acts on gold and makes it deposit. Now these two types are typical prevailing methods for electroless gold plating process.
In case of the electroless displacement gold plating process, the undercoat metal is displaced by gold deposition, and therefore the undercoat metal is dissolved (etched or eroded) as the gold is deposited. Particularly, when a thick-plated gold coating film is desired, the electroless displacement gold plating solution for thick-plating is used. In this case, the undercoat metal may severely be dissolved. This gives an adverse effect on the physical properties of the resulting gold coating film, such as adhesion, wire bondability, solder jointability, and solder wettability.
In order to prevent deterioration of physical properties of the coating, prior to thick electroless displacement gold plating, thin electroless displacement gold plating, and an undercoat plated coating film is formed on the undercoat metal to reduce the dissolution of the undercoat metal.
In this process, however, most of the undercoat metal surface is covered in the thin electroless displacement gold plating. Therefore, the undercoat metal can not be sufficiently dissolved in the thick electroless displacement gold plating, and the gold coating film can fail to reach the required thickness. Further, according to this process, the plated appearance may also be uneven in most cases.
In addition, as the thick electroless displacement gold plating, when the autocatalytic type electroless gold plating is used in which a reducing agent is made exist in a plating solution, the bath stability is poor, thereby causing many problems in practical use.
Accordingly, there has been a demand for development of an electroless displacement gold plating process that can provide a even plated appearance, a thick-plated coating, and good adhesion.